TITANUM OXIDE HYDROSOLS. SOME RELATIONSHIPS TO OXIDE HYDROSOLS OF ZIRCONIUM AND THORIUM HARRY S. OWENS A N D ROBERT MORRIS Department of Chemistry, University of Idaho, Moscow, Idaho Received January 16, 1998
Thomas and his coworkers have made numerous contributions to the literature of hydrous oxide hydrosols, explaining their reactions on the basis of polyolated structures. Since their work has been reviewed (10) recently it is unnecessary to discuss in this paper evidence in favor of the Thomas complex compound theory. However, two papers on basic thorium chloride (4) and basic zirconium chloride (6) hydrosols illustrate striking differences in the chemical reactions of these hydrosols when aged or when heated and then allowed to age; it seems advisable to mention these before giving the purpose of this investigation. Basic thorium chloride hydrosols age rapidly and come to equilibrium in two months. Boiling the freshly prepared sols increases the acidity to a relatively high degree, with equilibrium being reached in 10 hours. When such heated sols are cooled and allowed to stand a t room temperatures, there is a tendency for the acid produced to reverse the reaction so that the degree of acidity approaches the same values as those of the unheated sols aging a t room temperature. Basic zirconium chloride sols, on the other hand, age slowly but continuously for periods as long as eight months, if not longer. On boiling, the pH values decrease rapidly for a short time, then more slowly for at least 240 hours. The heated sols, when cooled and allowed to stand, remain at the pH values attained with no tendency to become less acid. These differences were ascribed to the less metallic nature of zirconium, which ionizes t o only a small extent as the quadrivalent ion and tends to form basic radicals. The oxygen bridges in these radicals are very resistant to the action of acids like hydrochloric acid, which contain anions having low penetrating power. Consequently there is little or no tendency for the chloride ion to penetrate the complex, thus breaking the oxo bridges, or for the hydrogen ion to unite with oxo bridges to form 01 groups.1 Titanium, occurring in a still lower position of the periodic table, is even Schmid (1) has found that in very concentrated solutions of hydrochloric acid some oxonium salts are formed with zirconium oxychloride, but he found little or no evidence for the formation of zirconium tetrachloride.
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less basic than zirconium. Therefore it seemed that it would form oxide hydrosols in which the micelles would contain mainly oxo bridges. The purpose of this paper is to submit evidence in support of this postulation. TABLE 1 Composition of hvdrosols SOL
T1. . . . . . . . . . . . . . . . . . . . . . . . . . . . T2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 1'3 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7
Ti
c1
120
9
13.4
7
6
PH 4
a
om~s
o m FlNAL
0.-
SALT
0.010
0,OlaS
0.018N
WNCCHTRATIOH
FIG.1. Effect of potassium salts on sol T2 EXPERIMENTAL
Preparation and composition of the hydrosols
Hydrated titanium oxide was precipitated from titanium chloride solutions by addition of a slight excess of ammonium hydroxide. Centrifugal washings and decantations were used until the oxide was free from chloride. 4 portion of this material was peptized in boiling dilute hydrochloric acid to prepare sol T2. Another portion was peptized in dilute hydrochloric acid at room temperature to prepare sol T3.
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Sol T1 was prepared by adding dilute ammonium hydroxide dropwise to a dilute solution of titanium tetrachloride until half the possible acid was neutralized. This sol was dialyzed for 148 hours. Table 1 shows the composition of these three sols, which are typical of the eight run during the course of the investigation.
Action of neutral salts The method used to study the effect of added neutral salts on the pH values of the hydrosols is essentially the same as that previously described (4). Figure 1 illustrates the results obtained with sol T2, which is typical of all those run. The order of effectiveness of the added anions to raise the pH value is oxalate > tartrate > sulfate > chloride. This order, while less complete, is the same as that obtained by Stewart (2). TABLE 2 Effect of heating titanium oxide hydrosols (b) sol T3
(a) sol T2 T I Y E OF E E A l l N Q
PH
TIME OF HEATING
hours
0 1 10 24 48
96
PH
hours
0 24 48
2.7 2.7 2.7 2.7 2.7 2.7
96
2.0 2.0 2.0 1.9
E$’ect of heating The effect of heating was studied in the same manner as with the basic zirconium chloride sols (6). The results are given in table 2. The effect of heating on the action of the sol with potassium sulfate is shown in figure 1. DISCUSSION
Titanium oxide sols react in some ways like basic zirconium chloride and basic thorium chloride hydrosols. For example, the order of effectiveness of anions in mising the pH values of the sols is the same for the few salts run. Also the tartrate, if allowed t o remain in contact with the precipitate originally produced on addition to the sol, will peptize the precipitate to foSm a titanate hydrosol. But there the similarities seem to end and differences are the rule. The effectiveness of the anions might be in the same order, but the magnitude of the effect is not nearly so great. If the increases in the pH values are compared with the results obtained by von Wicklen (8) with dilute
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hydrochloric acid and neutral salt solutions, one finds that there must be little displacement of hydroxo groups and most of the p H change comes from simple buffer action. This is borne out by experiments on heating in which the pH values of the sols remain practically constant even after 96 hours heating. If there were many hydroxo or 01 groups present, there should have been some tendency for the formation of oxo bridges through ordinary oxolation with the concomitant increase in acidity. The absence of any increase in acidity on heating, accompanied by a decrease in reactivity of the sol with neutral salts, indicates that oxo bridges are formed through the following reaction.
I I
-Ti-OH-Ti-OH
I II
--+
I I
-Ti-0-Ti-
I I
+ HzO
From the facts presented it seems logical to conclude that the micelles in these titanium oxide hydrosols are composed mainly of titanium oxides2 This conclusion agrees with the findings of Weiser and Milligan (9). However, even with these, the simplest of 'the oxide hydrosols yet studied from this point of view, the conclusion should not be drawn that chemical reactions are completely obviated. There is to be explained, besides the displacement of hydroxo groups, the reaction with tartrate forming negatively charged micelles. This is done nicely by assuming chelation, such as occurred with basic zirconium chloride (7), basic thorium chloride (5),and basic beryllium salt hydrosols (3). Stewart (2) in his investigation of titanium oxide hydrosols agrees with this explanation. SUMMARY
Micelles in titanium oxide hydrosols are mainly titanium oxide with few hydroxo or 01 groups in the complex. This conclusion is drawn from the small change in p H values obtained on adding neutral salts or heating titanium oxide hydrosols. The result is entirely in accordance with the Thomas theory of colloidal oxides when the position of titanium in the periodic table is considered. REFERENCES (1) SCHMID: Z.anorg. allgem. Chem. 167,369 (1927). (2) STEWART: Dissertation, Columbia University, 1937. (3) THOMAS AND MILLER:J. Am. Chem. SOC.68, 2526 (1936). (4) THOMAS AKD KREMER: J. Am. Chem. SOC.67, 1821 (1935). (5) THOMAS AND KREMER: J. Am. Chem. SOC.67, 2538 (1935). (6) THOMAS AND OWEKS:J. Am. Chem. SOC.67, 1825 (1935). (7)THOMAS AND OWENS:J. Am. Chem. Soc. 67, 2131 (1935). (8)V O N WICKLEN: J. Am. Leather Chem. Assoc. 29, 194 (1934). (9)WEISER AND NILLIGAN: J. Phys. Chem. 40, 1095 (1936). (10) WHITEHEAD:Chem. Rev. 21,113 (1937). 2 Water of hydration has not been considered, although it must be present to aliow irregular series formation with potassium tartrate.
